The vitreous fused silica ceramics plays a major role in aerospace applications. Amorphous form of fused silica is well known for interesting properties such as its very good thermal shock resistance, low and stable dielectric properties (dielectric constant and loss tangent) in the wide range of temperature, most abundant in nature, ease of fabrication, relatively cheap of all other ceramics suitable for radomes (e.g. alumina, boron nitride, mullite, silicon nitride etc). Because of these qualities, it is found suitable for radome applications over other ceramic materials to maintain mechanical strength at high temperature and low losses. Radome is a structural enclosure for radar antenna of ship, aircrafts or missiles providing good protection from hostile environments (e.g. aerodynamic heating, bird hits, rain erosion etc) without interference to the operation of the antenna. Modern high velocity aircrafts, missiles demands materials which can with stand high thermal shock, greater surface temperature, larger loading force and heavy rain erosion. These requirements can be met only by ceramic radomes.
It is noteworthy to mention here that fused silica has a tendency to de-vitrify and turns to crystalline phase (e.g. α-cristobalite) during sintering when excessive temperature and prolonged sintering is selected. This in turn transform into β-cristobalite structure while cooling to 180° C.-270° C. due to displacive transformation, which is common in silica ceramics. The volume change involved in this phase transformation leads to loss of strength and thermal shock resistance of the material. So it becomes necessary to incorporate crystallization control measures while sintering.
Slip casting of fused silica is a most viable process for radome production. Slip casting has its own advantages like cost effectiveness, simplicity, scalability and ease in handling of complex designs. In addition, slip cast fused silica displays relatively very less shrinkage in comparison to alumina or pyroceram which in turn favors thickness control measures. Accordingly, slip casting of fused silica is considered as a most prudent way for radome production and thus it has been most widely practiced till now. But due to moderate green packing density (around 75-80% of theoretical density) of slip-cast radomes, the sintered density will be mostly around 85-90% of theoretical density. Consequently, this porous nature of radome leads to water absorption problem which in turn deteriorates its property at operating conditions.
Various attempts have been made to overcome this problem. But, most of them lead to complex and costly processes which were not as fruitful as slip casting. Hence, it is essential to improve density and to retard devitrification for realizing high density slip cast fused silica bodies.
Prior art search was made in the patent and non-patent public literature to find out the works related to present invention. None of the prior-art literature discloses the method for developing high density slip-cast fused silica structures for radome application by adding Boron Oxide (B2O3) with the fused silica and using slip casting method for manufacturing fused silica bodies. However, the following literatures are referred due to their relevance to the field of present invention.
Reference may be made to Korean patent application number KR2007066729-A, wherein a fabrication process is disclosed for manufacturing high density and high fracture toughness radome using slip casting technique by addition of silica staple fiber to silica slurry. But maintaining uniform suspension of slip cast slurry during casting remains challenging which in turn complicates the process. Further it is difficult to maintain uniform dielectric loss in all direction and subsequently degrades the electromagnetic performance.
Reference may also be made to U.S. Pat. No. 6,091,375, which discloses radome with porous structure impregnated with glass, ceramics etc. It involves additional impregnating steps. However, it is difficult to ensure uniform impregnation. Further, thermal mismatch between matrix and impregnate at high temperature is not addressed properly.
Reference may be made to U.S. Pat. No. 4,949,095, wherein a new and improved high density fused silica radome is discussed. It describes that the radome is made by an arc fusion process, in which a quartz powder is placed within a graphite mold, and then shaped by centrifugal force as the mold is rotated. An arc is then struck between electrodes within the mold cavity. The quartz powder fuses to form a dense silica radome which is removed from the mold after the fusion occurs. Nonetheless, the said process of this is complex, expensive and is not suitable for large scale production.
U.S. Pat. Nos. 4,504,114 and 6,669,536 discuss about Boron doped fused silica for optical fiber applications. However, it mostly involves expensive vapor phase synthesis route which can only suit to perform preparation for optical fibers production.
From the above mentioned prior-art, it is understood that there is need for preparing high density fused silica bodies by simple, inexpensive route suitable for large scale production more particularly for radome fabrication. Density by slip casting as reported in prior-art is around 85% of theoretical density. This needs to be further improved in order to enhance the rain erosion resistance. Hence, there is a need to evolve a process for manufacturing the high density fused silica bodies by using slip-casting method.